1. Field of the Invention
The invention relates in general to a tilt control method of a near-field optical system.
2. Description of the Related Art
For the existing disc technology, the increase in storage capacity can be implemented by way of multi-layered rewritable discs or by reducing the size of the focused light spots for increasing the storage capacity per layer, hence increasing the total storage capacity of a disc. The size of the focused light spots is normally determined according to the wavelength of the laser light and the numerical aperture (NA) of the optical system. In general, a light source with short wavelength and an optical system with large numerical aperture can generate small focused light spots.
Due to the development in the increase of the storage capacity of the disc in recent years, the wavelength of the laser light has become shorter and shorter, and the laser light has been changed from the infra-red light laser (for use in CD) to the red light laser (for use in DVD), and further to the blue light laser. Since the blue light laser is used as the laser light, the related development in increasing the storage capacity of the disc is directed to reading/writing a disc with a near-field optical system for generating smaller focused light spots by increasing NA value. To increase the NA value, the near-field optical system normally generates focused light spots with an assembly of an object lens (convex lens) and a solid immersion lens (SIL). When reads/writes a disc with a near-field optical system, the gap between the SIL and the disc surface would become even smaller such as about 30-100 nm if a blue light laser is used, and such region is called near-field region.
Since the perpendicularity between the focused beam and the disc data layer affects the capability and accuracy of an optical disc drive in the read/write of data, the drive normally performs tilt control on the optical system (or the optical head) during the read/write of data, so that the focused beam is perpendicular to the disc data layer. However, the SIL and the disc surface are very close when a near-field optical system is used for reading/writing a disc. Therefore, how to perform the tilt control on the SIL and how to avoid the SIL colliding with the disc have become a prominent task for the industries.
However, before entering the near-field region, if the tilt angle between the SIL of the optical head and the disc is very large, the distance between the SIL and the disc might be erroneously judged. Under the case that the gap ranges 30-100 nm, the SIL may easily collide with the disc during adjustment of the tilt angle of the optical head. Thus, the accuracy of the near-field optical disc drive is negatively affected. Besides, for a generally known near-field optical disc drive, when the optical head is still within a far-field region (the gap between the SIL and the disc surface is larger than the near-field region), the position of the SIL can hardly be determined, and the tilt control cannot be performed on the optical head (or the SIL) within the far-field region.